The invention relates generally to the field of digital phase controllers, and more specifically to the field of direct digital phase control for resonant inverters, such, as for providing power to a lamp assembly.
High frequency resonant inverters are used in a wide range of applications requiring high quality ac output, including electronic ballasts for lighting, induction-based heating appliances, medical equipment, and semiconductor processing equipment. In most applications, a closed-loop controller is required to regulate the output waveforms for improved rejection of component tolerances and variations in environmental conditions. Common approaches for regulating the output include control of the input dc bus, duty cycle, switching frequency, or triac cut-in angle.
Frequency control is one of the most popular control schemes due to its simplicity and wide dynamic range. However, direct frequency control results in a number of disadvantages, including high sensitivity near resonance, strong dependence on resonant tank component values, and a requirement for additional control circuitry to detect operation below resonance to prevent non-ZVS (zero-voltage-switching) conditions. Another alternative control method in electronic ballasts is to regulate the phase angle between mid-point voltage of a half-bridge inverter and the resonant inductor voltage or current. Phase control provides the advantages of self-tuning relative to the tank resonant frequency (insensitive to component variations), reduced sensitivity for improved control near resonance, and inherent protection against operation below resonance to avoid non-ZVS conditions.
Integrated circuit (IC) implementations of analog phase and frequency controllers have been known in the art to achieve the benefits of reduced external component count. In addition, a digital frequency controller for electronic ballasts has been developed, but does not include an approach for phase control with a direct digital interface to advanced controllers.
It is therefore desirable to provide for direct digital phase control of resonant inverters that achieves the benefits of temperature and semiconductor process independence, rapid design cycles, and real-time re-programmability in the controller operation.